1. Field of the Invention:
The invention relates in general to feedback control for turbine-driven high-powered AC synchronous generator systems and in particular a means and method for reducing the noise generated in the means for sensing torsional oscillation in the mechanical portion of the power generating system.
2. Description of the Prior Art:
In long line electrical transmission systems of the type where the electrical generators are driven from power sources such as steam turbines, multiple rotating masses may be present on a single shaft, such as, the rotating mass of one or more turbines and the rotating mass of an exciter upon AC generator as well as the rotating mass of the generator rotor. Because of the mechanical properties of the various parts of the rotating system, torsional oscillation may be possible between any combination of masses. It has been found in certain applications that such undamped torsional oscillations may exist because of the electrical properties of the AC synchronous generator electrical system. The inductive effect of the long transmission lines may require a series capacitive compensation and the effect of the compensating capacitors being connected in series circuit relationship with the inductance of the long transmission lines tends to cause certain resonant frequencies to exist in the output electrical circuit of the AC generator. These resonant frequencies may be reflected by the magnetic coupling between the stator and rotor of the synchronous generator to the mechanically rotating shaft. Should these electrical resonant frequencies coincide with a critical mechanical resonant frequency between any of the masses of the generating system, reinforced undamped oscillation may occur which may be of sufficient magnitude to break the shaft. Apparatus and method for solving this problem have been proposed in U.S. Pat.No. 3,999,115 entitled "Dynamic Stabilizer For Synchronous Machines Having Torsional Oscillations And Method," issued on Dec. 21, 1976 to William H. South and assigned to the same assignee as the present invention. This patent teaches method and apparatus that sense torsional oscillation and quickly operate on the output power of the synchronous generator to thus provide compensation. The torsional oscillation is sensed by a speed sensing or tachometer means which is provided to sense and measure the speed of the rotating shaft in a turbine generator system of the type previously described and provide a signal related thereto. This signal then is provided to a compensating means for compensating for the modulation of the electrical output power by the torsional oscillation within the generator system. One of the problems involved with this arrangement for providing compensation to the output power of the synchronous generator is that the sensing means used to sense the speed of the rotating shaft is subject to several sources of error which can cause a velocity deviation signal (noise). That is, the extracted signal can indicate that a velocity deviation exists on the shaft, when in fact, the generator system is rotating at synchronous speed with no outside torsional disturbances. The velocity deviation signal (noise) may be equal or even exceed the signal due to the torsional oscillation of the shaft. Attempts to filter out the undesirable noise signals from the sensing means signal may add a phase shift delay to the output signal from the sensing means which phase shift is undesirable since either an active or passive compensating means must provide an output that is 180.degree. phase shifted from the synchronous generator to thus provide compensation. It would be advantageous if a method and apparatus could be found which would reduce the unwanted noise in the output of the sensing means signal to a large degree while providing a minimal phase shift to that portion of the sensing means signal which is passed through to the compensating means.
One speed sensing means includes a variable reluctance electromagnetic transducer which is mounted close to a multiple toothed machined tooth gear wheel. One of the problems associated with the machining of tooth gear wheels is to determine the tooth to tooth distances and the tooth widths of the gear after the machining process. The present method utilizes a mechanical system which measures the amount of pressure with "master" gear with known accuracies, i.e. errors in runout, tooth width, etc., and the chart recorder must be synchronized to the master gear. However this system is subject to the same errors associated with any mechanical means. In addition, the same master gear must be available at all times to insure repeatable measuring results. Accordingly it would be desirable if an electromechanical means could determine the machining accuracy of a tooth gear wheel since this would not be subject to the disadvantages of the mechanical system currently used.